The central characters of this story are Mary Maloney and her husband, Mr. Maloney. We actually are told far more about Mary Maloney so I will focus on her in this response, as she is by far the more interesting character.

This story is frequently taught in schools as an excellent example of irony, and what is absolutely key to this is how Dahl builds up his picture of her as a loving wife. Consider how she is first introduced:

Now and again she would glance up at the clock, but without anxiety, merely to please herself with the thought that each minute gone by made it nearer the time when he could come. There was a slow smiling air about her, and about everything she did.

Note too how her actions are stereotypical of a loving wife: she greets her husband with a kiss, takes his coat, makes him a drink. Note how Dahl continues to develop this image of her as a loving, perfect wife:

She loved to luxuriate in the presence of this man, and to feel - almost as a sunbather feels the sun - that warm male glow that came out of him to her when they were alone together. She loved him for the way he sat loosely in a chair, for the way he came in a door, or moved slowly across the room with long strides.

This is almost an obsessed kind of love but it serves to set the stage for the situational irony of what is to come. When Patrick Maloney tells her that he is leaving him, she strikes him on the head with a leg of lamb and then shrewdly engineers the removal of the murder weapon and thus all evidence of her crime. Such an act is unexpected and at variance with the image of her that we are led to believe at the beginning of the story, and perhaps suggests the darker message of the story - that love and hate are not so strictly separated after all and that a thin dividing line is all that separates them.